1) deletionists 2) secret editor cabals
Wikipedia is stunningly open to new editors, unless you create a page that someone else doesn't like. I've seen multiple people get turned off from Wikipedia when they create two or three pages and each gets hit with a speedy deletion within days.
The other big problem with Wikipedia, for slightly more advanced editors, is that the ethics of the site frown on recruiting allies for revision wars...and yet there is tons of evidence that there's a vast world of behind-the-scenes editor politics, factions, rivalries, and Joe User can make a change, or weigh in on a controversy, and a dozen editors can jump in at once.
After being burned a few times I've avoided the cliquey corners of wikipedia and mostly just work on things like spacecraft docking standards...but not everyone is content to work on out of the limelight areas.
I think it was Clay Shirky who once said that it was a mistake to try to solve social problems with technological fixes.
I think Wikipedia's big problems are social problems.
(Disclaimer: it's still an amazing site, and I'm a big fan...but it could be better.)